I. Field
The present document relates generally to wireless communication and amongst other things to handoff in a wireless communication system.
II. Background
An orthogonal frequency division multiple access (OFDMA) system utilizes orthogonal frequency division multiplexing (OFDM). OFDM is a multi-carrier modulation technique that partitions the overall system bandwidth into multiple (N) orthogonal frequency subcarriers. These subcarriers may also be called tones, bins, and frequency channels. Each subcarrier is associated with a respective sub carrier that may be modulated with data. Up to N modulation symbols may be sent on the N total subcarriers in each OFDM symbol period. These modulation symbols are converted to the time-domain with an N-point inverse fast Fourier transform (IFFT) to generate a transformed symbol that contains N time-domain chips or samples.
In a frequency hopping communication system, data is transmitted on different frequency subcarriers in different time intervals, which may be referred to as “hop periods”. These frequency subcarriers may be provided by orthogonal frequency division multiplexing, other multi-carrier modulation techniques, or some other constructs. With frequency hopping, the data transmission hops from subcarrier to subcarrier in a pseudo-random manner. This hopping provides frequency diversity and allows the data transmission to better withstand deleterious path effects such as narrow-band interference, jamming, fading, and so on.
An OFDMA system can support multiple mobile stations simultaneously. For a frequency hopping OFDMA system, a data transmission for a given mobile station may be sent on a “traffic” channel that is associated with a specific frequency hopping (FH) sequence. This FH sequence indicates the specific subcarrier to use for the data transmission in each hop period. Multiple data transmissions for multiple mobile stations may be sent simultaneously on multiple traffic channels that are associated with different FH sequences. These FH sequences may be defined to be orthogonal to one another so that only one traffic channel, and thus only one data transmission, uses each subcarrier in each hop period. By using orthogonal FH sequences, the multiple data transmissions generally do not interfere with one another while enjoying the benefits of frequency diversity.
An accurate estimate of a wireless channel between a transmitter and a receiver is normally needed in order to recover data sent via the wireless channel. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. The pilot signal is made up of pilot symbols that are known a priori by both the transmitter and receiver. The receiver can thus estimate the channel response based on the received symbols and the known symbols.
A code division multiple access (CDMA) system has a universal frequency reuse that makes it possible for mobile users to receive and send the same signal simultaneously from and to multiple base stations or sectors of a base station. Soft and softer handoff in CDMA systems are techniques whereby mobiles near cell, and sector in the case of softer handoff, boundaries communicate the same transmitted signals to more than one base station or sector of a base station. Soft and softer handoff provides enhanced communication quality and a smoother transition compared to the conventional hard handoff. Soft and softer handoff is intrinsic to a CDMA system, as transmitted signals of different users occupy the same time and frequency allocation. Different users can be separated based on the respective spreading signatures.
Supporting soft and softer handoff in orthogonal multiple-access systems such as TDMA, FDMA and OFDMA is far more difficult and often requires special planning. Consider a reverse link transmission in FH-OFDMA. Each user is assigned a non-overlapping time and frequency resource. As such there is little or no intra-cell. However, it is often not possible to reliably detect the signal in a nearby sector or cell, as the interference is considerably large compared to the signal. Low signal-to-noise ratio causes the channel estimation to be inaccurate, further degrading the overall detection performance. Often, the post-detection signal-to-noise ratio (SNR) is too low for the signal observed in a nearby cell/sector to be useful. Techniques such as active set based restricted frequency (ASBR) hopping and common hopping sequence can be used to help improve the detection reliability of the signal observed in a nearby sector/cell. These techniques, however, result in smaller usable system resources (e.g., bandwidth) and often require significant planning.
Therefore, there is a need to find efficient approaches to provide soft and softer handoff in OFDMA systems while minimizing the amount of overhead required to perform the soft and softer handoff.